司馬昭之心路人皆知

1. • lit. Sima Zhao's intentions are obvious to everyone (idiom)
2. • fig. an open secret

Character by character

• 司(sī)surname Si
• 馬(mǎ)surname Ma
• 昭(zhāo)bright
• 之(zhī)(possessive particle, literary equivalent of 的 de)
• 心(xīn)heart
• 路(lù)surname Lu
• 人(rén)person
• 皆(jiē)all
• 知(zhī)know